Numerous devices and methods have been proposed and many of them used in the prior art for working and/or testing rubber and other elastomers, e.g., to test rheological, elastic and related properties in test samples of such materials. Standard test procedures have been established by certain agencies, such as the American Society for Testing Materials, the U.S. Bureau of Standards, and various persons and agencies in the rubber and plastic industries. The known procedures are of various types and have been applied in different ways. Among these, the more successful appear to be those in which a sample, e.g., a disc of the material is worked or tested by driving either a rotating or oscillating disc which is embedded in the sample of material. Temperature control means, including heating elements, are incorporated in the dies to (1 ) permit the measurement of viscosity, and/or (2) promote vulcanization in those materials which cure when subjected to elevated temperatures. In one of the methods proposed to test vulcanizable materials, one of the dies is oscillated angularly with respect to the other about this common axis. This applies a shear stress or a working to the material; in the usual case, the internal resistance of the sample to such working increases towards a desired or predetermined maximum and the torque applied to oscillate the die, which reflects the degree of curing, etc., is measured by suitable sensing devices to provide a record of the procedure and its effect on the sample. Some devices are useful only for testing viscosity in solid and semi-solid elastomers, etc. Some of the latter are driven in continuous rotation. These may be very satisfactory for determining viscosity of raw or compounded rubbers at low temperatures but are not desirable for testing vulcanizable materials at curing temperatures because the sample materials would be rapidly destroyed in the test procedure.
Hence, in testing vulcanizable elastomers, it has been usual practice to grip the sample between opposed axially aligned dies and then to oscillate one of the dies or a biconical disc embedded in the sample, through a small angle or rotation about the common die axis, applying a shear stress first in one direction and then in the other, successive applications of the stress passing through zero. In general, the prior art discloses a non-rotatable die, usually the upper, which can be raised or lowered to release or grip the sample against a lower die which, in some cases, is mounted to be oscillated by a motor driven lever or rocker arm. Heating means, such as electrical resistance elements, are usually incorporated in the dies for heating the sample to curing temperature and for controlling the temperature throughout the test.
The devices just mentioned all have one important deficiency. The degree or amplitude of shear applied is a maximum at the periphery of the sample and zero at its center; hence, the sample is worked unevenly. There is a tendency to tear the sample loose from the dies at its outer edges, whereas the central part is hardly worked at all. Also, in the case of the oscillating disc cure meter the disc not only acts as a heat sink and retards the cure, but the twist in the disc shaft and yielding of the torque arm results in a strain loss of up to 50 percent with some rubbers. A primary object of the present invention is to overcome these deficiencies, using an apparatus and method which in some respects is quite similar to those of prior art but in other respects is very different.
Some workers in the prior art have recognized the deficiencies mentioned above but they have not succeeded in eliminating them. It has been proposed, for example, to reduce the angle of oscillation to reduce the strain loss and slippage at the periphery of the sample. While this is partially effective for the purpose stated, it reduces even further the degree of working accomplished at and near the center of the sample. Thereby it limits the maximum torque (resistance to working) in the sample and renders the curemeter or testing device less sensitive to minor changes in the elastomer than it would be if a larger angle of oscillation, and consequently a larger energy input, could be tolerated. See the 1974 book of ASTM standards, Method D2084, for example.
The deficiencies mentioned above, and others inherent in the prior art systems contribute to difficulties in obtaining good graphical records or representations, as pointed out, for example, in the patent to Wise, U.S. Pat. No. 3,387,490, mentioned above. For this reason, Wise has proposed a complication in recording that would desirably be avoided. See also, an article by Decker et al. in Rubber World, December, 1962, where recording problems are discussed further. Briefly stated, when the stress applied is rapidly reversed, passing through zero repeatedly, as noted above, peaks or spikes are shown which confuse rather than elucidate the results being achieved. The true state of curing, for example, must be indicated in such cases by a true envelope of the oscillating peaks, and must be interpolated manually or mentally or else a complicated recording system must be resorted to. In contrast, according to the present invention, a smooth, definitive curve, properly indicative of actual results, is readily obtained.
The present invention is based to a large degree on the discovery by the present inventor that the above deficiencies can be largely or entirely eliminated by changing the sample working or shear-stressing operation to a simple, continuous, non-reversing gyratory action. In this system, one die, e.g., the lower, is mounted for relatively free rotation, with respect to its driver, on a continuously driven rotary driver whose axis is eccentric to, i.e. offset a small distance from the axis of the upper die. With the sample gripped by its more or less plane faces between the relatively fixed die and the gyrating die, the latter is resiliently restrained from actual rotation while being carried in a small-diameter circle around the axis of the upper die; the lower face of the sample is stressed in shear continuously and all of its area, transversely, is subjected to the same amplitude of distortion or stress. More energy can be put into the working without damaging any part of the sample due to overworking, and the sensitivity of the test is greatly improved. The tests can be carried out more rapidly and/or curing is faster than with the oscillating disc curemeter because the elimination of the conventional disc greatly improves heat transfer.